Lec 20

• Inbreeding


  Mating of animals more closely related than the average of the population
• Identical by descent


  Having alleles originating from the same individual
• Many would not want to inbreed because inbreeding has been associated with reduced fitness and fertility (Inbreeding Depression)
• Inbred animals

  Prepotent - producing uniform offspring with a high degree of predictability
• Linebreeding


  A system of mating which maintains a substantial degree of relationship to a highly regarded ancestor while keeping inbreeding to a low level
• Linebreeding


  A milder form of inbreeding
• Linebreeding keeps inbreeding to less than 12.5%
• If we breed a sire back to his daughter or mating full sister to full brother, we can expect inbreeding level to reach 25%
• Coefficient of relationship (R)

  Degree of kinship, R > 0.25 to a single common ancestor
• Effects of linebreeding


  • Having a higher probability of acquiring common genes derived from a common ancestor
  • Individuals are related or identical by descent to the common ancestor
• Purposes of linebreeding

  • To develop consistency and uniformity within a herd or breed
  • To fix desirable alleles associated with superior phenotypes derived from an outstanding individual/ancestor
  • To uncover and eliminate harmful and undesirable recessive alleles within the population
  • To develop prepotent animals desirable in the development of distinct family lines
• Strain


  A sub-population within a breed that develops into a variant of the established breed
• Linebreeding can lead to loss of general vigour, especially reduced reproductive performance
• Selection should accompany linebreeding to reduce the effects associated with inbreeding
• With linebreeding we get a set of uniform offspring with both desirable and undesirable alleles in their genomes
• Need to do annual selection to keep the desirable phenotypes as parents and cull the undesirable offspring
• Example of Progressive Retinal Atrophy (PRA) in dogs

  • Dd x Dd mating of 2 heterozygotes, carriers
  • DD - 1 normal, not affected
  • Dd - 2 carriers
  • dd - 1 affected, blind
• Linebreeding aims to increase homozygosity for all genes with desirable characteristics from a common ancestor, while keeping inbreeding level low
• Linebreeding is able to set particular characteristics in a homozygous form to develop a strain within a breed that will show uniformity and consistency